Indium (In) is a rare, soft and malleable post-transition metal which is chemically similar to gallium (Ga) and has an atomic number of 49 and an atomic weight of 114.8. It has a low melting point (156.6° C.); a property which makes it useful in various low melting point applications, such as being a component in low melting point lead-free solders. In appearance, indium is a silvery-white true metal with a bright luster and emits a high-pitched “cry” when bent. Furthermore, it has a standard electrode potential of +0.34V and three oxidation states (+3, +2, and +1), the most common being+3. In nature indium is usually found in solid solution of some zinc sulphide (sphalerite) minerals.
Indium is commonly used to form transparent electrodes from indium tin oxide in liquid displays and touchscreens. It is also used in thin-films to form lubricated layers, for example, in high-performance bearing applications. In a radioactive form Indium-111 is used in leukocyte imaging for tracking leukocytes movement in living organisms.
Gallium (Ga), as noted above, is similar in characteristics to indium, being a group 13 post-transitional metal. Gallium has an atomic number of 31 and an atomic weight of 69.7. Furthermore, gallium has an extremely low melting point of 29.7° C. and has a brilliant silvery colour. Gallium has three oxidation states (+3, +2, and +1), the most common of which being +3. The electrode potential of gallium is +0.56V. Interestingly, it does not exist in free form in nature and is often found as a trace metal in bauxite, some sphalerite and compounded with copper in gallite (CuGaS2).
Gallium is commonly used as part of the compound gallium arsenide in microwave circuitry and infrared applications as a well as gallium nitride, and gallium indium nitride in semiconductor applications. It is also used in blue and violet light-emitting diodes and diode lasers.
With its various uses and owing to supply and demand concerns, recently the world price of indium has increased, in some instances by 7 to 10 times in the past 5 years. Also, the cost of gallium and the fact that it does not exist in a free form in nature cause it to have a relatively high price of about $220/100 g. As a result, recovery of the indium and gallium content from various sources is desirable.
One such source from which indium and gallium can be obtained is from recycling or waste material from the construction of copper-indium-gallium (CIG), copper-indium-gallium-selenium (CIGS), gallium-indium-zinc oxide (GIZO) and indium-gallium-selenium (IGS) solar panel cells. During the manufacturing process of the CIG, CIGS, GIZO and IGS solar panels, scraps and waste material are generated which contain indium and gallium in large quantities. Also, other metals, such as selenium, which may be harmful to the environment if not disposed of properly, or which may also be desirable to recover, may be found in this scrap or recyclable material.
With a current shift in many regions of the world to alternative energy sources used or worn-out solar panels such as those described above will become an increasing problem. Also, scrap material from the manufacturing process of new solar panels can be recycled and thus diverted from waste facilities. Therefore, it would be advantageous and desirable to develop a method for recovering indium and gallium, as well as other metals from waste material from solar panel production and from the recycling of used solar panels.